Split KIdentityManagement into a core and widgets library

We have many more users of the core classes than the configuration UI, including many of the background processes. For those we now avoid linking against a bunch of unnecessary and not exactly lightweight UI libraries, such as the entire rich text editing stack.

Most consumers are unaffected by this change, some notice the lack of KPIMTextEdit in the public link interface (already fixed), MRs for the repos actually using widgets will follow shortly.